Collaboration and License Agreements - Collaboration Revenue and Research and Development Expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenue Recognition and Deferred Revenue [Abstract]
Collaboration revenue – related party	$ 8,732	$ 10,643	$ 42,478	$ 32,497
Reduction of research and development expenses	$ 0	$ 1,078	$ 0	$ 6,343